SOILS 

BY 

HENRY  W.  NICHOLS 
Associate  Curator  of  Geology 


Geology 

Leaflet  5 


FIELD  MUSEUM  OF  NATURAL  HISTORY 
CHICAGO 

192S 


LIST  OF  GEOLOGICAL  LEAFLETS  ISSUED  TO  DATE 

Model  of  an  Arizona  Gold  Mine $  .10 

Models  of  Blast  Furnaces  for  Smelting  Iron     .        .10 
Amber  -Its  Physical  Properties  and 

Geological  Occurrence 10 

Meteorites 10 

Soils 10 

D.  C.  DAVIES,  Director 


FIELD  MUSEUM  OF  NATURAL  HISTORY 
CHICAGO.  U.  S.  A. 


No. 

1. 

No. 

2. 

No. 

8. 

No. 

4. 

No. 

5. 

FRONTISPIECE.        RESIDUAL  SOIL. 
In  the  tropics  soil  forms  to  srreat  depths.      The  excavation  and  mine  opening 
shown  here  are  all  in  soil,  formed  by  the  disintegration  of  rock  in  place. 
Brejauba,  Brazil. 
Capt.  Marshall  Field  Geological  Expedition  to  Brazil,  1922. 


Field  Museum  of  Natural  History 

DEPARTMENT  OF  GEOLOGY 
Chicago,  1925 

Leaflet  Number  5 


SOILS 

It  is  a  matter  of  ordinary  observation  that  the 
hard  rocks  of  the  land  surface  of  the  earth  are  nearly 
everywhere  concealed  by  a  covering  of  loose  materials 
of  which  the  sands,  gravels  and  clays  are  the  most 
abundant  and  familiar.  This  covering  is  composed  of 
fragments  and  other  wastes  from  the  disintegrating 
rock  surfaces.  It  may  be  of  any  thickness  from  a 
fraction  of  an  inch  to  hundreds  of  feet.  It  is  upon  the 
upper  surface  of  this  unconsolidated  material  that 
ordinary  vegetation  takes  root  and  grows.  In  most 
places  this  surface  has  been  so  modified  by  exposure  to 
air  and  weather  and  by  the  effects  of  growing  and 
decaying  vegetation,  that  it  is  much  better  able  to  sup- 
port vegetation  than  is  the  unaltered  material  below. 
This  modified  surface  is  the  soil.  The  unmodified 
material  below  is  the  subsoil.  Soil  is  ordinarily  shallow 
and  varies  in  depth  from  a  fraction  of  an  inch  to  a 
few  feet  at  most.  In  many  arid  regions  and  perhaps 
in  some  other  places  there  is  no  perceptible  difference 
between  the  surface  which  should  be  the  soil  and  the 
subsoil  below.  In  such  cases  the  surface  is  spoken  of 
as  the  soil  and  the  distinction  between  soil  and  subsoil 
is  not  made.  In  regions,  however,  where  the  distinction 
between  soil  and  subsoil  is  readily  made,  a  surface 
from  which  the  soil  has  been  removed  either  by  natural 
or  artificial  means  is  not  ordinarily  spoken  of  as  the 

146] 


2  Field  Museum  of  Natural  History 

soil  merely  because  it  happens  to  be  at  the  surface. 
Such  places  in  time  develop  a  new  soil  to  cover  them. 

FORMATION  OF  THE  SOIL. 

Soil  is  the  product  of  the  breaking  down  of  solid 
rock.  It  contains  fragments  from  the  mechanical  dis- 
integration of  rock  and  insoluble  residues  left  from  the 
chemical  decomposition  and  solution  of  rock.  With 
these  are  mingled  smaller  quantities  of  organic  matter, 
detritus  from  the  decay  of  vegetation.  Rock  is  dis- 
integrated by  various  mechanical  agencies  which  pro- 
duce fragments  of  all  sizes  from  large  boulders  to  dust 
particles  of  minutest  size.  The  larger  fragments, 
boulders,  pebbles,  gravels  and  sands  are  familiar  to  all. 
Besides  the  larger  fragments  quantities  of  rock  pow- 
der and  dust  called  rock  flour  are  made.  In  many 
places,  as  around  Chicago,  this  rock  flour  forms  a  not 
inconsiderable  part  of  the  soil.  Those  geological 
agencies  which  disintegrate  are  numerous  and  are  con- 
tinuously at  work.  Important  among  them  is  the 
action  of  frost.  Water,  filling  crevices  and  pores  in  the 
rock,  freezes  and  in  freezing  expands  and  wedges  apart 
and  breaks  off  fragments.  Running  water  is  another 
effective  agent.  Sand  and  gravel  dragged  over  a  rock 
bottom  by  the  current  of  a  stream  grind  away  the  rock 
at  a  fairly  rapid  rate.  Boulders  and  pebbles  grinding 
against  each  other  as  they  are  carried  down  stream 
by  a  rapid  current  are  reduced  to  smaller  size  by 
abrasion.  Wave  action  upon  a  pebble-covered  beach 
has  much  the  same  effect.  Where  the  rock  surface  is 
not  covered  by  soil,  expansion  and  contraction  due  to 
differences  in  temperature  between  day  and  night, 
cause  the  rock  surface  to  disintegrate.  There  are  many 
other  sources  of  rock  disintegration. 

In  glacial  regions,  among  which  northern  Illinois 
is  included,  rock  detritus  from  glaciation  has  a  pro- 

[46] 


Soils  3 

found  effect  upon  the  character  of  the  soil,  for  in  such 
regions  the  soil  is  largely  composed  of  rock  detritus 
of  glacial  origin.  This  material  is  the  result  of  the 
grinding  away  of  the  surface  rock  by  the  scouring 
action  of  moving  bodies  of  ice,  which,  filled  with  sand 
and  gravel  at  their  bases,  acted  much  like  gigantic 
pieces  of  sand  paper. 

ROCK  DECOMPOSITION.  Plate  I.  The  dura- 
bility of  the  stone  fronts  of  our  buildings  would 
suggest  that  rock  is  weather  proof  and  unalterable. 
Such  is  not  the  case.  The  impression  of  durability  is 
due  in  part  to  the  favorable  position  of  stone  in  build- 
ings and  in  part  to  the  short  lapse  of  time  since  even 
the  first  building  was  erected  compared  with  the  much 
longer  periods  during  which  the  soil  has  been  forming. 
The  principal  agent  involved  in  the  decomposition  of 
rock  is  a  very  mild  one,  being  merely  rain  and  surface 
water.  If  this  water  were  pure  it  would  have  little 
effect  upon  rock  but  its  action  is  intensified  by  the 
effect  of  gases  which  it  absorbs  from  the  air  and  by 
other  substances  which  it  absorbs  from  the  soil  and 
acquires  in  other  ways. 

When  rock  decomposes  a  number  of  products  are 
formed.  Some  of  these  dissolve  in  water  and  are 
washed  away.  Others  are  insoluble  and  remain.  In 
spite  of  the  varied  composition  of  the  different  rocks, 
these  insoluble  residues  are,  when  considered  in  a 
general  way,  surprisingly  alike.  Disregarding  frag- 
ments of  undissolved  rock  which  are  commonly  present, 
the  residues  are  sands  and  clays.  The  sands  are 
particles  varying  in  diameter  from  one  twenty-fifth  of 
an  inch  to  sizes  barely  visible  to  the  unaided  eye.  These 
are  almost  always  fragments  of  the  mineral  quartz, 
one  of  the  commonest  rock  minerals  and  one  decidedly 
resistant  to  solution  or  alteration.  Clay,  the  other 
important  product  of  rock  decomposition,  is  a  very 

[47] 


4  Field  Museum  of  Natural  History 

finely  divided  material  largely  composed  of  compounds 
of  silica  and  alumina  combined  with  water,  of  which 
the  mineral  kaolin  is  typical.  Most  of  the  rock-form- 
ing minerals  which  are  readily  decomposed  contain 
silica  and  alumina  in  quantity.  From  these  substances 
combined  with  water,  kaolin  and  a  number  of  similar 
minerals  which  together  form  the  basis  of  clay  are 
formed.  With  these  minerals  there  are  usually  present 
in  clay  a  variety  of  others  which  assume  clay-like 
properties  because  they  are  in  the  form  of  exceedingly 
minute  particles. 

ROCK  SOLUTION.  Some  rocks,  such  as  the 
limestones,  are  slowly  soluble  in  water  and  under 
favorable  conditions  are  completely  removed  in  solu- 
tion. Such  rocks  contain  small  quantities  of  insoluble 
materials  as  impurities.  When  the  rock  is  dissolved 
these  remain  in  the  form  of  clay. 

TRANSPORTATION.  Rock  detritus  may  remain 
in  the  place  where  it  was  formed.  It  is  then  called  a 
residual  soil.  A  residual  soil  from  the  solution  of  lime- 
stone is  a  clay  which  is  usually  very  fertile.  Residual 
soil  from  the  decomposition  of  other  rocks  (Frontis- 
piece) often  attains  great  depths  in  the  tropics.  Often 
the  change  from  soil  at  the  surface  through  the  sub- 
soil to  the  solid  rock  is  so  gradual  that  there  is  no  per- 
ceptible line  of  separation  between  subsoil  and  rock. 
In  such  soils  the  structure  and  general  appearance  of 
the  parent  rock  are  often  preserved.  More  frequently 
soils  have  been  transported  and  sorted  by  running 
waters,  the  ice  of  the  glacial  period  or  even  by  winds. 
By  these  means  the  sands,  clays  and  gravels  have  been 
more  or  less  assorted  to  mixtures  which  are  quite  dif- 
ferent from  those  originally  formed.  Soils  are  some- 
times classified  according  to  the  ways  in  which  this 
transportation  and  redeposition  has  been  accomplished. 
Thus  we  have  besides  numerous  others,  alluvial  soils 

[48] 


PLATE   I.        FORMATION   OF  SOIL   FROM    IGNEOUS   ROCK. 

The  series  shows  gradual  rounding  of  angular  blocks  until  the  entire  rock  has 
disintegrated  to  soil.  »_,_•.»        „ 

Brighton,  Mass. 


Soils  5 

deposited  from  water  and  glacial  soils  transported  by 
the  ice  of  the  glacial  period. 

COMPOSITION  OF  SOIL. 

Soils  are  essentially  mixtures  of  some  or  all  of  the 
following:  Gravel,  sand,  clay  and  organic  matter  in 
the  form  of  humus.  Carbonate  of  lime  either  as  pul- 
verized limestone,  or  in  the  form  of  shells  or  other 
organic  residues  is  sometimes  present  in  quantity. 
Upon  the  proportions  and  nature  of  these  several  com- 
ponents, the  general  character  of  the  soil  depends. 
While  other  substances  present  in  small  quantities  have 
a  profound  effect  upon  soil  fertility,  circulation  of  air 
and  water,  ability  to  retain  plant  foods  and  such 
important  matters  are  largely  determined  by  the  pro- 
portions of  these  major  components  present. 

GRAVEL.  Plate  II.  Gravel  is  composed  of  frag- 
ments of  the  rocks  from  which  the  soil  has  been 
derived.  In  the  gravel  is  locked  up  much  of  the  future 
fertility  of  the  soil,  to  be  given  out  by  degrees  as  the 
pebbles  decompose.  Too  large  a  quantity  of  gravel 
causes  sterility  by  reducing  the  quantity  of  the  more 
essential  soil  components.  When  the  rock  fragments 
of  a  gravel  are  less  than  about  one  twenty-fifth  of  an 
inch  in  diameter  the  material  is  called  a  sand. 

SAND.  Plate  III.  Rock  fragments  smaller  than 
one  twenty-fifth  of  an  inch  in  diameter  but  large 
enough  so  that  the  individual  grains  may  be  readily 
distinguished  by  the  naked  eye,  form  sand.  Sands 
found  in  soils  are  usually  composed  chiefly  of  the  min- 
eral quartz,  so  much  so  in  fact,  that  sands  are  often 
spoken  of  as  if  they  were  wholly  quartz.  Yet  practically 
all  sands  contain  particles  of  other  minerals  than  quartz 
and  sometimes  the  other  minerals  form  a  large  part  of 
the  substance  of  the  sand.  There  are  also  sands  from 
which  quartz  is  nearly  or  quite  absent.    The  specimen 

[49] 


6  Field  Museum  of  Natural  History 

shown  in  the  soil  collections,  while  predominantly  of 
quartz,  contains  quantities  of  limestone,  feldspar,  mica, 
oxides  of  iron  and  other  minerals.  The  sands  about 
Chicago  contain  important  quantities  of  limestone 
particles  and  generally  grains  of  oxide  of  iron,  The 
presence  of  sand  in  quantity  causes  a  soil  to  be  light, 
that  is  to  work  easily,  be  porous  and  consequently  dry. 
As  such  a  soil  allows  water  to  penetrate  freely,  soluble 
plant  foods  may  be  washed  away,  causing  sterility.  As 
air  penetrates  between  the  sand  grains  readily,  a  soil 
with  much  sand  is  seldom  sour,  as  the  organic  acids 
formed  by  the  decomposition  of  organic  matter  are 
rapidly  oxidized  and  destroyed  by  the  action  of  the  air. 

CLAY.  Plate  IV.  This  is  a  finely  divided  plastic 
mixture  of  minerals.  Its  most  important  constituents 
are  hydrated  silicates  of  alumina  of  which  the  mineral 
kaolin  is  the  most  typical  and  abundant.  These 
silicates  are  derived  from  the  decomposition  of  the 
feldspars  and  other  aluminous  minerals  of  the  parent 
rock.  As  clay  is  tenacious  and  absorbs  water  readily, 
it  makes  a  soil  heavy,  that  is,  resistant  to  implements 
of  cultivation  and  to  the  passage  of  water.  The  absence 
of  porosity  makes  circulation  of  air  through  clay  diffi- 
cult so  that  soils  containing  much  clay  readily  become 
sour  for  the  organic  acids  produced  by  the  decomposi- 
tion of  organic  matter  are  not  rapidly  destroyed  by 
oxidation.  Clay  has  the  important  property  of  retain- 
ing from  waters  passing  through  it,  ammonia,  phos- 
phoric acid,  potash  and  other  valuable  plant  foods. 

Soil  in  which  the  individual  grains  are  nearly  or 
quite  imperceptible  to  the  naked  eye  but  are  larger 
than  the  impalpably  fine  particles  of  a  clay  is  called 
silt.  Silt  is  intermediate  in  character  between  clay 
and  sand,  but  as  it  is  more  or  less  clay-like  in  character 
according  as  its  particles  are  finer  or  coarser  it  may 
be  regarded  for  most  purposes  as  an  imperfect  or 
impure  form  of  clay. 

[60] 


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Soils  7 

CARBONATE  OF  LIME.  Carbonate  of  lime, 
which  is  present  in  small  quantities  in  most  soils  and 
in  large  amounts  in  some,  has  a  beneficial  effect  upon 
the  fertility  of  the  soil.  Some  authorities  say  that  lime 
is  the  substance  which  holds  commanding  importance, 
aside  from  physical  conditions,  in  controlling  fertility. 
It  is  sometimes  in  the  form  of  fragments  and  powder 
from  broken-down  limestone  and  sometimes  in  the 
form  of  shells  or  other  organic  remains.  Its  presence 
makes  a  soil  more  open  and  porous  and  it  assists  in 
holding  plant  foods.  There  is  from  one  to  three  per 
cent  of  it  in  ordinary  soils  and  some  marls  contain  as 
much  as  ninety  per  cent. 

HUMUS.  Humus  is  the  product  of  decaying  or- 
ganic matter.  It  comes  from  vegetation  which  has 
grown  on  the  soil  in  which  it  is  found.  It  gives  soil 
a  dark  color  and  is  a  very  important  ingredient  of  fer- 
tile soils,  as  it  provides  the  nitrogen  necessary  for 
vegetable  growth  and  is  also  rich  in  the  mineral  plant 
foods.  It  is  found  in  ordinary  soils  only  in  small  quan- 
tities but  forms  half  the  bulk  of  some  vegetable  mold. 
Under  certain  conditions  frequently  encountered,  some 
of  the  organic  matter  of  humus  assumes  an  acid  char- 
acter, making  the  soil  sour.  A  sour  soil  is  decidedly 
unfavorable  to  the  growth  of  most  crops.  This  sour- 
ness is  counteracted  by  farmers  in  a  number  of  ways 
— such  as  adding  lime  to  neutralize  the  acid,  exposure 
to  strong  sunlight,  drainage  and  treatments  increasing 
the  circulation  of  air  in  the  soil. 

Many  other  substances  are  present  in  soils  in 
small  quantities  and  some  of  these  have  considerable 
effect  in  increasing  or  decreasing  fertility. 

PLANT  FOODS. 

While  plants  secure  much  of  their  food  from  the 
air,  some  mineral  matter  and  some  forms  of  organic 

[51] 


8  Field  Museum  of  Natural  History 

matter  must  be  secured  from  the  soil  in  which  they 
grow.  Many  of  these  mineral  plant  foods,  such  as  lime 
and  oxide  of  iron,  are  practically  always  present  in 
sufficient  quantity.  But  certain  others  of  the  greatest 
importance  are  removed  from  the  soil  by  cultivated 
crops  faster  than  the  decomposition  of  the  soil  particles 
can  provide  them.  Hence  their  presence  and  quantity 
is  a  matter  of  vital  importance  to  the  farmer.  These 
foods  are  listed  below. 

POTASH  SALTS.  These  are  derived  from  the 
decay  of  potash-bearing  minerals,  such  as  the  feldspars 
of  the  rocks  from  which  the  soils  were  formed.  The 
rock  fragments  composing  the  soil  continue  to  decom- 
pose and  gradually  the  potash-bearing  minerals  they 
hold  give  up  potash  in  the  form  of  soluble  compounds. 
These  may  be  (1)  absorbed  by  vegetation,  (2)  stored 
in  the  soil,  particularly  if  the  soil  is  clayey,  or  (3) 
washed  away  and  wasted. 

PHOSPHATES.  These  plant  foods  also  come  from 
the  parent  rock  of  the  soil.  However,  phosphate-bear- 
ing minerals  are  by  no  means  as  abundant  in  the  rocks 
as  are  potash-bearing  minerals,  so  that  much  of  the 
phosphate  used  as  plant  food  comes  from  the  humus 
products  of  the  decay  of  former  vegetation.  The  same 
thing  is  true,  probably  in  lesser  degree,  of  potash  com- 
pounds. This  means  the  same  phosphate  and  potash 
is  used  over  and  over  again  by  succeeding  generations 
of  plants.  As  a  consequence,  when  land  which  in  its 
wild  state  was  very  fertile  has  been  cultivated  for  some 
years,  the  fertility  diminishes,  for  the  potash  salts  and 
phosphate  which  were  continually  returned  to  the  soil 
when  the  land  was  wild  have  been  taken  permanently 
from  the  soil  as  the  crops  were  gathered.  Fresh  sup- 
plies of  these  foods  from  the  decay  of  soil  particles 
are  not  provided  rapidly  enough  to  compensate  for  the 
drain  upon  the  supply.    Therefore,  if  land  is  under 

[52] 


Soils  9 

constant  cultivation,  it  is  necessary  to  replenish  these 
plant  foods  from  time  to  time  by  the  application  of 
fertilizers. 

NITROGEN.  Nitrogen  is  another  element  of 
plant  life  which  must  be  provided  from  the  soil.  Most 
of  it  is  in  the  form  of  humus  from  previous  vegetation. 
Important  quantities  are  also  provided  by  certain 
bacteria  which  abstract  and  fix  nitrogen  from  the  air. 
Also  some  ammonia  and  acid  compounds  of  nitrogen 
are  formed  from  the  nitrogen  of  the  air  during  thunder 
storms  and  are  washed  into  the  soil  by  rain  water. 
Like  the  potash  and  phosphates,  the  nitrogen  of  a  soil 
is  depleted  by  the  removal  of  crops  and  must  be 
replaced  by  the  application  of  manures  and  fertilizers 
or  by  the  growing  of  crops  which  promote  the  growth 
of  the  nitrogen-fixing  bacteria. 

OTHER  PLANT  FOODS.  There  are  a  number  of 
other  mineral  plant  foods  which,  as  already  stated,  are 
practically  always  present  in  sufficient  quantity  for 
plant  growth.  These  include  lime,  iron,  silica  and 
others  which  are  either  needed  only  in  small  quantity 
or  are  always  present  in  abundance.  The  application 
of  lime  to  a  soil  is  seldom  necessary  because  of  an 
insufficient  supply  for  plant  food,  but  rather  to  modify 
the  physical  condition  of  the  soil  or  to  render  other 
plant  foods  more  readily  available. 

SOIL  POISONS. 

Substances  are  sometimes  present  in  soils  which 
greatly  diminish  their  fertility  or  even  induce  com- 
plete sterility.  On  the  beaches  and  marshes  of  the  sea- 
shore, ordinary  vegetation  cannot  grow  owing  to  the 
presence  of  salt.  A  special  vegetation  that  has  devel- 
oped toleration  for  salt  to  so  high  a  degree  that  it  can- 
not exist  without  it  takes  the  place,  therefore,  of 
ordinary  upland  vegetation.    Soluble  alkali  salts  which 

[53] 


10  Field  Museum  of  Natural  History 

are  often  present  in  the  soils  of  arid  and  semi-arid 
regions  have  the  same  effect  on  plant  growth  as  the 
salt  of  the  sea-shore.  Another  kind  of  soil  poison 
which  has  been  recognized  in  recent  years  is  that 
believed  to  be  produced  by  poisonous  secretions  from 
growing  crops  which  interfere  with  the  subsequent 
growth  of  the  same  crops  on  the  same  land. 

CLASSIFICATION  OF  SOILS. 

A  number  of  soil  classifications  are  in  use  and  are 
adapted  to  different  methods  of  soil  study.  For  ordi- 
nary use,  where  exhaustive  study  of  the  subject  is  not 
contemplated,  a  classification  which  has  arisen  through 
centuries  of  agricultural  observation  is  universally 
employed  in  farming  regions.  This  separates  soils  into 
groups  which  are  determined  principally  by  the  sizes 
of  the  soil  particles.  This  classification  recognizes  four 
fundamental  classes,  gravels,  sands,  loams  and  clays, 
to  which  are  added  two  other  groups,  marls  and 
vegetable  molds  on  account  of  the  great  influence  on 
the  fertility  of  soils  which  lime  and  organic  matter 
exercise. 

GRAVEL  when  pure  consists  of  rock  fragments 
over  one  twenty-fifth  of  an  inch  in  size.  As  gravels  are 
absolutely  sterile,  they  are  not,  strictly  speaking,  soils 
at  all. 

SANDY  SOILS  consist  essentially  of  sand  with 
which  may  be  mixed  small  quantities  of  vegetable  mold 
and  clay.  The  pure  sands,  as  those  of  many  beaches 
and  dunes,  are  practically  sterile,  so  that  they  may  be 
considered  as  soils  only  when  some  humus  or  clay  is 
present.  The  humus  of  such  sandy  soils  provides  and 
the  clay  retains  plant  foods  so  that  such  soils  may  be 
fairly  fertile.  However,  as  the  open  texture  of  the 
sands  is  such  that  water  circulates  readily,  plant  foods 
are  quickly  leached  out  and  therefore  such  soils  tend 

[54] 


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Soils  11 

to  be  sterile.  As  such  soils  allow  water  to  drain  away 
readily  they  do  not  retain  moisture  and  this  in  most 
climates  is  unfavorable  for  fertility.  The  porosity 
of  sandy  soils  promotes  the  rapid  circulation  of  air 
which  oxidizes  any  organic  acid  present  in  the  humus 
so  that  such  soils  are  seldom  sour.  As  there  is  no 
cohesion  between  the  sand  grains,  sandy  soils  yield 
readily  to  agricultural  tools  and  are  easily  worked. 
They  are  often  called  light  soils,  not  because  they 
weigh  little,  but  because  they  are  easily  worked.  When 
a  soil  contains  less  than  twenty  per  cent  of  sand,  the 
properties  of  clay  begin  to  be  noticeable  and  the  soil 
becomes  a  loam. 

LOAM.  When  a  soil  composed  largely  of  sand 
yet  contains  sufficient  clay  so  that  the  properties  of 
clay  become  apparent,  the  soil  is  a  loam.  A  loam 
therefore  presents  some  of  the  qualities  of  both  sandy 
and  clayey  soils.  It  has  in  lesser  degree  the  porosity 
of  sandy  soils  and  yet  shows  a  little  of  the  tenacity  and 
plasticity  of  clay  soils.  The  loams  are  our  most  valu- 
able soils.  They  have  porosity  so  that  they  are  usually 
kept  sweet  by  circulating  air  and  yet  the  pores  are 
sufficiently  stopped  by  clay  so  that  water  does  not 
drain  off  readily  and  therefore  they  are  not  rapidly 
impoverished  by  leaching  of  the  plant  foods  nor  do 
they  dry  out  as  rapidly  as  the  sands  in  times  of 
drought.  Loam  particles  possess  some  cohesion  from 
the  clay  present  but  this  cohesion  is  not  sufficient  to 
make  the  soil  heavy  to  work.  Loam  soils  may  be 
divided  into  the  two  groups  of  sandy  loams  and  clay 
loams. 

No  exact  percentage  of  sand  and  clay  can  be  stated 
which  separates  a  loam  from  a  sand  or  clay  soil,  for 
the  distinction  is  based  upon  a  combination  of  the 
effects  of  sand  and  clay  upon  the  properties  of  the  soil. 
As  sands  are  coarser  or  finer  they  are  able  to  impress 

[66] 


12  Field  Museum  of  Natural  History 

their  character  upon  the  soil  more  or  less  strongly. 
Clays  also  vary  in  physical  character  so  that  the  same 
amount  of  different  clays  will  affect  a  soil  differently. 

CLAY  OR  HEAVY  SOILS.  These  soils  are  called 
heavy  because  they  offer  much  resistance  to  the  tools 
used  in  cultivation.  They  are  composed  of  clay  with 
some  sand  and  humus.  These  soils  in  their  natural 
state  tend  to  be  wet  and  sour  and  hence,  sterile.  The 
tenacity  of  the  clay  makes  tillage  difficult.  Properly 
cultivated  they  are  often  very  fertile.  They  are  less 
easily  exhausted  by  cultivation  than  most  soils. 

MARLS.  Plate  V.  Soils  containing  much  lime 
are  distinguished  from  other  soils  by  the  name  marl. 
This  distinction  is  made  because  of  the  noticeable  effect, 
usually  beneficial,  of  carbonate  of  lime  on  soil.  So 
great  is  the  modification  of  soil  by  lime  that  marls  high 
in  lime  are  applied  to  other  soils  as  fertilizers.  A  shell 
marl  is  a  marl  in  which  the  carbonate  of  lime  is  present 
in  the  form  of  shells.  Marls  are  light,  porous  soils 
which  do  not  become  sour  and  retain  plant  foods  well. 
They  frequently  contain  some  of  the  plant  foods  in 
unusual  quantity. 

HUMUS  SOILS.  Soils  colored  by  more  than  five 
per  cent  of  humus  form  a  very  fertile  group  of  soils. 
They  include  vegetable  mold  and  muck. 

Vegetable  mold  is  a  thin,  black  soil  formed  in 
forests  and  elsewhere  by  the  decay  of  vegetation. 

Muck  is  a  soil  formed  in  wet  places  by  the  arrested 
decay  of  vegetation  in  the  presence  of  water.  In  their 
natural  state  mucks  are  wet  and  sour  but  when  prop- 
erly prepared  for  cultivation  they  are  of  unusual  fer- 
tility. 

The  simple  soil  classification  above  given  has  been 
elaborated  by  the  United  States  Department  of  Agri- 
culture into  a  complex  system  suited  for  use  in  the 

[56] 


<    S 


Soils  13 

intensive  study  of  soils  by  precision  methods.  A  col- 
lection of  soils  arranged  according  to  this  more 
elaborate  system  is  shown  in  the  Museum  in  a  separate 
case. 

NOTE. 

The  soil  is  absolutely  essential  for  human  life  and 
progress.  Consequently  it  has  been  much  studied  and 
much  is  known  about  it.  The  more  essential  parts  of 
this  knowledge  are  of  such  a  nature  that  they  can  be 
illustrated  by  specimens  and  the  Museum  has  pre- 
pared collections  to  illustrate  these.  They  show  the 
origin,  nature,  composition  and  kinds  of  soil  and  some 
other  features  such  as  plant  food  in  soil.  Other  often 
important  facts  concerning  soils  are  of  such  a  nature 
that  they  can  not  be  well  illustrated  by  specimens.  This 
leaflet  treats  of  only  such  aspects  of  the  soil  as  the 
collections  illustrate.  The  Museum  soil  collection  is 
exhibited  in  the  southwest  quarter  of  Hall  36  on  the 
second  floor  of  the  building. 


[67] 


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